The field of the invention relates to speed control systems for motor vehicles.
Speed control systems are known which control the engine throttle by feedback control from an error signal derived by subtracting vehicle speed from a reference speed. Such speed control systems typically include both an acceleration mode and a resume mode. During the acceleration mode, the reference speed is continuously incremented during operator depression of an acceleration switch. The resume mode reactivates speed control operation which was previously disengaged in response to vehicle braking. Upon operator actuation of a resume switch, the reference speed stored during previous speed control operation is utilized as a new reference speed.
Operator perceived problems of lurching, speed overshoot and speed undershoot may exist with some prior speed control systems during both the acceleration mode and the resume mode. Although these conditions do not present any safety hazard, they may be unpleasant to the operator. For example, upon initiation of a resume mode, the difference between vehicle speed at initiation and the previously stored reference speed may create a speed error signal of sufficient magnitude to cause a momentary lurch in vehicle speed. Further, this acceleration will likely cause some speed overshoot of the reference speed at termination of the resume mode.
The above problem is addressed by U.S. Pat. No. 4,337,511 issued to Schneider et al. In response to a resume command, a reference speed is gradually ramped up from vehicle speed existing at occurrence of the resume command to the desired speed. A series of two reference ramps are also shown in an attempt to more gradually approach the desired speed to minimize speed overshoot.
The inventors herein have recognized numerous disadvantages of the prior art acceleration and resume modes. For example, use of a reference ramp or series of reference ramps typically assumes flat road conditions and a known vehicle response. For example, when the vehicle is traveling on a downgrade and initiates a resume mode, vehicle momentum will cause the vehicle speed to exceed the reference ramp. The resulting negative speed error will cause the speed control system to turn the engine throttle towards idle. After the downgrade is traversed, the negative speed error will then cause the vehicle to abruptly slow down, and undershoot the desired speed. Speed control systems having integral feedback which accumulate a negative speed error while traversing a downgrade may have a more pronounced undershoot under such conditions.
The inventors herein have recognized a similar disadvantage of prior approaches when the operator intervenes by depressing the accelerator during a resume mode. More specifically, the resulting excess of vehicle speed over the reference ramp causes a negative speed error. The speed control system turns the engine towards idle in response to the speed error. As the desired speed is approached, the operator will cease accelerating where upon a negative speed error may cause an abrupt vehicle deceleration and speed undershoot of the reference speed.